Upcoming emission regulations for diesel engines are driving manufacturers to incorporate after-treatment devices in the exhaust systems in order to comply with these regulations. These devices could include: Diesel Particulate Filters, Oxidation Catalysts, NOx Adsorbers, SCR Catalysts, etc. Due to the nature of operation, diesel engines have a relatively low exhaust gas temperature. This low exhaust gas temperature presents a challenge to the after-treatment devices that tend to operate more efficiently within a temperature window. During a cold start emission test, the engine exhaust gas remains below the temperature operating range of the after-treatment devices for a long period of time resulting in exhaust gas that was not treated being emitted out the tailpipe. These emissions can contribute significantly to the measured regulated emission data. In some instances during normal operation (such as idle conditions or low speed/load points) the temperature of the exhaust gas can drop below the operating window of the after-treatment device. In order to fully realize the potential of the after-treatment device, the temperature would need to be maintained inside the temperature window. Thus, a way of increasing the temperature of the device is needed.
Several different approaches have been investigated including: post injection of fuel in cylinder to create an exotherm, burner systems, and electrically heated catalysts. Post injection of fuel in the cylinder has had some success in raising exhaust gas temperature in combination of using an oxidation catalyst to create an exotherm but has limitations on cold start due to a higher catalyst light off temperature. This method does have limitations in that the fuel consumption can be large due to combustion of the fuel in the cylinder where little hydrocarbons reach the oxidation catalyst to create the required exotherm. Another drawback of this type of system is the potential for cylinder wall wetting creating oil dilution in the engine leading to more frequent oil changes and potential engine failure. Burner type systems have been effective in heating but require complex ignition and control systems as well as additional hardware added to system. Electrically heated catalyst systems require large amounts of electrical energy resulting in large fuel economy penalties. Also, as the engine size goes up, so does the exhaust flow rate resulting in even higher amounts of energy consumed to increase the temperature of the exhaust gas.